Research ArticleDesign and Performance Research of Flexible Drill Pipe JointBased on Bionic Theory

Chuanliu Wang and Pei Ju

Xi’an Research Institute of China Coal Research Institute, Xi’an 710077, China

Correspondence should be addressed to Chuanliu Wang; wangchuanliu@cctegxian.com

Received 12 May 2021; Revised 23 June 2021; Accepted 20 July 2021; Published 4 August 2021

Academic Editor: Xudong Zhang

Copyright © 2021 Chuanliu Wang and Pei Ju. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Fatigue fracture is the main failure form of mining drill pipe joints, which directly leads to the occurrence of drill pipe dropaccidents. Based on the principle of engineering bionics, the spine structure and kinematic behavior characteristics of cheetahwere studied; the bionic design and trial production of drill pipe joints were carried out. For the bionic drill pipe joint, itseparates the torsion transmission function from the connection function, and when the torsion transmission function of thejoint fails, the connection function is still effective, so as to achieve the effect of “breaking the bones and connecting the ribs.”The mechanical and flow field properties of the bionic drill pipe joint were analyzed. Mechanical test results showed that, for thebionic drill pipe joint, the average tensile capacity was 555.48 kN and the average torsion capacity was 8914.13N·m, which metthe mechanical performance requirements. The flow field numerical simulation results showed that, when the drilling fluidvelocity field and pressure field were stable and change evenly, there is no risk of leakage. The study results can provide a newresearch idea for the research and development of mine drill pipe joint with long life and antidropping function.

1. Introduction

Drilling technology is widely used in the fields of gas drainageand water hazard control in coal mines, which is of great sig-nificance to the safe production of coal mines. During thedrilling process in coal mine, the force of the drill pipe jointis complicated, and due to the limitation of the space size ofunderground roadway, there has more threaded connectionsfor the drill pipes, which leads to the rigidity of the pipe isstrong, and the fracture accidents of the drill pipes occurfrom time to time [1]. According to statistics, more than75% of drill pipe fractures are located at the root of the exter-nal thread (as shown in Figure 1). The drill pipe is severelybent in the hole, and stress concentration is likely to occurat the root of the external thread, which leads to the fractureof the drill pipe and the formation of the “drop drilling” acci-dent. When the “drop drilling” accident occurs, it is neces-sary to carry out fishing operations, which will waste a lotof manpower and material resources and increase the drillingproduction cycle and cost [2–6]. Therefore, the key to solving

the “drop drilling” problem is to solve the problem of thefracture at the root position of the external thread.

To solve the above problem, scholars have proposedmany solutions, which can be summarized in three aspects.(1) Newmaterials: research the materials with better compre-hensive performance and improve the overall performance ofexternal threads. For example is He who took 4145Hmaterialand φ88:9mm drill pipe joint thread (NC38) as the researchobject, and he applied the nanocopper antifriction coatingAFRICO to the joint thread, so as to achieve the purpose ofimproving the antisticking performance of the thread [7].(2) Structural size optimization: optimize the overall struc-ture of drill pipe, especially the size of the thread, and addstress grooves. For example is Tian et al. who studied thestrength and sealing characteristics of different thread struc-tures, such as round thread, trapezoidal thread, and partialtrapezoidal thread, and they pointed out that the doubleshoulder thread structure was helpful to protect the overload[8]; Liu et al. proposed a method of setting TLM stress reliefgroove in the internal thread of the upper joint of drill pipe,

HindawiGeofluidsVolume 2021, Article ID 9661388, 9 pageshttps://doi.org/10.1155/2021/9661388

so as to reduce the frequency of drill pipe leakage and threadfracture effectively [9]. (3) New type of thread: develop newtooth profile, optimize thread parameters, perform surfacetreatment, etc. For example is Gao et al. who designed anew type of V-50R thread profile structure, which canimprove the torsional strength of the thread and alleviatethe problem of stress concentration by increasing the pitchand side angle of the thread [10]; Dong et al. designed anew type of W-038R thread profile structure, which canreduce the stress concentration at the bottom of the threadby increasing the thread root size and bottom arc radius;finally, the torsion and bending resistance of the thread canbe improved [11]. The core of the above measures is to reducethe stress concentration at the root of the external thread, butit does not essentially solve the problem of drill pipe “dropdrilling” caused by the fracture of the external thread.

Bionics, as an emerging edge subject arising from theorganic combination of biological sciences and technical sci-ences, can produce unexpected innovations when applied tothe field of drilling technology [12–15]. Based on this,inspired by the strong structure of four-legged mammalssuch as cheetahs, the cheetah spine structure is used as abionic model, and the drill pipe joint is designed as bionicstructure. By using laser cutting and alloy die-castingmethods, the drill pipe joint is processed into a combinationof three parts: the torsion shoulder, the cushion layer, and thefront thread, which can achieve the effect of “breaking thebones and connecting the ribs.” By means of laboratory testand numerical simulation, the mechanical and flow filed per-formance of the bionic drill pipe joint is analyzed, and thereliability of the design is explained, so as to explore a new

research idea for solving the problem of drill pipe “dropdrilling.”

2. Bionic Design of Drill Pipe Joint

The spine of the animal is composed of vertebrae and inter-vertebral discs and is connected by elastic cartilage and liga-ments. It is a very soft and flexible structure with multipledegrees of freedom [16]. This paper takes cheetah as theresearch object and provides a biological prototype for thebionic design of drill pipe joint through the study of its spinalstructure and movement behavior characteristics.

2.1. Spinal Structure andMovement Behavior of Cheetah. Thespine of cheetah is mainly composed of cervical spine, tho-racic spine, and lumbar spine, as shown in Figure 2. The tho-racic structure is composed of thoracic spine, sternum, andribs, which can protect the heart, lungs, and other importantorgans, so for the thoracic structure, the stiffness is greater.The lumbar spine is more flexible and can carry out up anddown, left and right bending and axial torsion movements,so for the lumbar spine structure, the stiffness is relativelysmall. The vertebrae and intervertebral disc of the cheetahare chimeric with each other, and there are some structuralfeatures such as soft connection between elastic cartilage andligament along the circumferential direction, which can pro-vide inspiration for the bionic design of drill pipe joint [17, 18].

In order to describe the movement behavior of cheetah, amovement coordinate system as shown in Figure 3 is estab-lished. When the cheetah runs, the curvature of the spinemainly occurs in the lumbar spine, and the motion of the

Figure 1: Photos of fracture of the external thread of drill pipe.

Sacral spine Lumbar spine �oracic vertebra Cervical spine

SternumRib

Figure 2: Cheetah spine skeleton diagram.

2 Geofluids

second lumbar joint close to the thoracic spine is the largest.When the cheetah runs in a straight line, the curvature of itstrunk mainly occurs in the sagittal plane of the spine, whilethe curvature in the cross section and the horizontal planeis small; when the cheetah runs in a turn, the curvature ofthe spine mainly occurs in the horizontal plane.

2.2. Mathematical Model of the Movement of Spine. Combin-ing engineering bionics techniques and similar principles, thepseudo-rigid model is used to construct the mathematicalmodel of the cheetah’s spine, and by revealing the principleof the strength and toughness of the cheetah spine, provideguidance for the bionic design of the drill pipe joint.

According to the structural characteristics of the spine ofcheetah and referring to the anatomical structure parameterssuch as the length ratio relationship between vertebrae andintervertebral disc, the bionic joint is designed. The bionicjoint is composed of multiple combination segments (shownas Figure 4), and each combined segment can be regarded asa mechanical pseudo-rigid body model with 1 pseudo-elasticbody and 2 rigid bodies. Assume the width of the pseudo-elastic body is lp, the width of the rigid body is lg, the charac-teristic radius coefficient of the model is γ, the vertical loadapplied to the end of the spine is F, and the stiffness of thepseudo-elastic body is K1, K2,⋯, Kn. According to the prop-erties of pseudo-elastomeric materials, the characteristicradius coefficient γ, the parameter angle coefficient C0, andstiffness coefficient K0 can be obtained by linear interpolationmethod. The stiffness Hi of a certain combined segment canbe expressed as follows:

Hi = E + 2GS2i� �

I, ð1Þ

Si =R2lpi

, ð2Þ

where E is the Young model of the pseudo-elastomeric mate-rial, G is the shear modulus of the pseudo-elastomericmaterial, I is the inertia of cross-sectional moment of thepseudo-elastomeric body, Si is the shape factor of the ith

pseudo-elastomeric body, R is the effective diameter of thespine, and lpi is the thickness of the ith elastomer.

The stiffness of the pseudo-elastic body of each pseudo-rigid body model is expressed as follows:

Ki =γK0Hi

lpi: ð3Þ

The length of each pseudo-rigid body model is as follows:

Li = γlpi + lg i+1ð Þ + 1 − γð Þlp i+1ð Þ, i = 1, 2,⋯, n − 1,Li = γlpi + lg i+1ð Þ, i = n:

(

ð4Þ

The bending moment balance expression of each pseudo-rigid body model is as follows:

Kiθi = Ki+1Cθθi+1 + FLiCθ cos 〠i

j=1θj

!

, i = 1, 2,⋯, n − 1,

Kiθi = FLiCθ cos 〠i

j=1θj

!

, i = n,

8>>>>><

>>>>>:

ð5Þ

Cross section

Sagittal plane

Horizontal plane

Roll

Pitch

Deflection

Z

XY

Figure 3: Spine skeleton diagram of cheetah.

lg

lp

Figure 4: Pseudo-rigid body model of bionic joint.

3Geofluids

where θi and θj are the bending angles of the ith and jthpseudo-elastic bodies, respectively.

By solving Equation (5) numerically, the bending angle ofeach point can be obtained. Using these angle values, the totaldeformation of the bionic joint can be calculated as follows:

D = 〠n

i=1Li sin 〠

i

j=1θj

!

: ð6Þ

The overall bending stiffness of the bionic joint can alsobe obtained as follows:

KS =FL: ð7Þ

2.3. Bionic Design of the Structure of Drill Pipe Joint. Based onthe above analysis of the characteristics of cheetah spinalstructure, the idea of “softness overcoming rigidity” isadopted, the fragile (rigid) part of the root of drill pipe jointis cut into two parts (the torsion shoulder (the “1” part shownin Figure 5) and the front thread (the “2” part shown inFigure 5)), and then the cushion (flexible) layer (the “3” partshown in Figure 5) is injected into the gap between the tor-sion shoulder and the front thread; finally, an elastic cartilageand ligament structure of the drill pipe joint which is similarto the spine is formed, as shown in Figure 5.

The existence of the flexible cushion 3 is conducive toabsorbing certain deformation, improving the stress state ofthe drill pipe joint. Stress is distributed evenly along the slitcutting seam (shown as Figure 6), which can avoid the stressconcentration, and also, the force of the bionic joint varieswith the bending direction of the drill pipe. When the drillpipe joint is stressed heavily, the cushion layer is the first partto be destroyed, so that the broken part of the drill pipechanges from randomness to fixedness. Most importantly,the torque transmission function and the connection func-tion of the drill pipe joint are separated with this bionic struc-ture; when the joint torque transmission function fails, theconnection function is still effective, so as to achieve the effectof “breaking the bones and connecting the ribs.” The drillpipe still has high tensile strength, and the drill rod can besmoothly lifted out of the hole, thereby completely solvingthe problem of drill pipe “drop drilling” caused by the frac-ture of the drill pipe joint.

2.4. Processing of Bionic Drill Pipe Joint. The external joint ofthe drill pipe is cut along the fragile part on the root of thejoint by laser cutting method. The cutting path is petal-shaped; the torsion shoulder and the front thread are formedat the joint because of the existence of the slit, and these twoparts can be separated but cannot be taken off from eachother. Then, the copper-nickel alloy material is filled intothe cutting seam by high-temperature die-casting method,the cushion layer is formed, and the width of the cushionlayer is between 0.6 and 1.2mm.

For the filled copper-nickel alloy material, its yieldstrength is between 50 and 80% of the yield strength of thejoint material, the elongation is not lower than that of the

joint material, the elastic modulus is between 40 and 60% ofthe elastic modulus of the joint material, and it can withstanda certain amount of compression and tension deformation.

Finally, the circular turning and thread finishing are car-ried out, and nitriding is carried out to improve the hardnessand wear resistance of the joint surface. Figure 7 shows theprocessing flow of the bionic drill pipe joint.

3. Mechanical Performance of Bionic DrillPipe Joint

Based on the above analysis, the bionic drill pipe joint wasmachined, and the mechanical properties of the bionic drillpipe joint were tested.

3.1. Tensile Performance Testing. After the cutting of theouter joint of the drill pipe was completed, and before the

1

3

2

Figure 5: Schematic diagram of bionic structure of drill pipe joint.

Figure 6: Schematic diagram of stress distribution along the slit.

Slit by laser Filing cushion Machining thread

Figure 7: Processing flow of the bionic drill pipe joint.

4 Geofluids

alloy material was filled, the tensile strength of the drill pipewas tested by the WAW-1000 hydraulic proportional controluniversal testing machine (Figure 8). The average tensileforce was 555.48 kN, and the tensile strength of the drill pipebody met the performance requirements.

3.2. Torsion Resistance Testing. Torsion test on the outer jointof the bionic drill pipe was carried out by the torque test sys-tem (Figure 9). The average torsion breaking force was8914.13N·m, and the torsion resistance of the bionic drillpipe joint met the performance requirements.

4. Flow Field Performance Analysis of theBionic Drill Pipe Joint

Since the external joint of the bionic drill pipe is composed ofthree connecting components (the torsion shoulder, the frontthread, and the cushion layer), when the drilling fluid is flow-ing, there may have the risk of fluid puncture and leakage onthe connecting parts of the three components. So, the flowfield numerical simulation is carried out, and the flow velocityand pressure distribution of the fluid are analyzed to illustratethe reliability of the flow field of the bionic drill pipe joint.

00 2 4 6 8 10 12 14 16

Displacement (mm)18

50100150200250300

Forc

e (kN

)

350400450500550600650

Figure 8: Tensile performance test of the bionic drill pipe.

00 100 200 300 400

Time (s)500

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Torq

ue (N

. m)

Figure 9: Torsion resistance test of the bionic drill pipe.

(a) Coupling model of bionic drill pipe joint

InletOutlet

z

XY

(b) Fluid domain model

Figure 10: Bionic drill pipe joint model and fluid domain model.

5Geofluids

4.1. Numerical Model and Boundary Condition. The outerdiameter of the drill pipe is φ73mm, and the diameter ofthe inner through-hole of the outer joint is φ23mm.Figure 10 shows the bionic drill pipe joint coupling model

and the fluid domain model. In order to facilitate the numer-ical calculation, the fluid area is simplified, and the smallerchamfer at the mating surface is ignored to improve the qual-ity of meshing. Tetrahedral elements are used to mesh the

12

3

3.06e+01

Contour-2Velocity magnitude

Vector-1Velocity magnitude

2.75e+012.45e+012.15e+011.84e+011.54e+011.23e+019.30e+006.26e+003.22e+001.78e–01

(m/s)

3.30e+012.97e+012.64e+012.31e+011.98e+011.65e+011.32e+019.92e+006.62e+003.32e+002.48e−02

(m/s)

Figure 11: Cloud and vector diagrams of drilling fluid velocity in the axial cross section of the coupled model.

140 0.025 0.05 0.075 0.1

Position (m)0.125 0.15 0.175 0.2 0.225

16

18

20Velo

city

(m/s

)

22

24

26

28

30

Figure 12: Velocity curve of drilling fluid along the central axis of the coupled model.

6 Geofluids

model; in order to improve the calculation accuracy, themesh size is set to 1mm. The specific meshing model isshown in Figure 10(b).

Setting the flow rate of the mud pump is Q = 25 L/s, therotation speed is n = 100 rpm, the density of the drilling fluidis ρ = 1070 kg/m3, the viscosity of the drilling fluid is μ =0:00321 kg/ðm · sÞ, and the inner diameter of the drill pipeat the entrance is d = 47mm, so the flow velocity of the dril-ling fluid at the inlet boundary is v = 4Q/πd2 = 14:4m/s.

The Reynolds number is used to judge the flow state of thedrilling fluid in the drill pipe. The Reynolds number is calcu-lated as Re = ρvd/μ = 225600, so the flow state of the drillingfluid is turbulent; turbulence intensity can be expressed asI = 0:16 Re−1/8 = 0:034, turbulence scale is l = 0:07d =0:00329m, turbulent kinetic energy is k = 1:5ðvIÞ2 = 0:36m2

/s2, turbulence dissipation rate is ε = Cu3/4ðk3/2/lÞ = 10:79m2

/s3 (where Cu is an empirical constant, approximately 0.09).According to the flow characteristics, the k-ε turbulence

model and the scaled wall function considering the rough-ness of the side wall are selected, the calculation is based onthe pressure-based solver, and the least square interpolationcalculation based on the element volume is used [19]. Theinlet boundary condition is set as the velocity inlet, the outletboundary condition is set as the pressure outlet, and the out-let pressure is set to 1.0MPa.

4.2. Analysis of Simulation Results. Based on the symmetricalfeature of the drill pipe joint structure, the simulation resultsare analyzed by intercepting the center cross section alongthe Y-axis of the coupling model.

Figure 11 shows the drilling fluid velocity cloud and vec-tor diagram along the Y-axis center cross section of thebionic drill pipe coupling model. The drilling fluid entersthe bionic joint from the left side at a velocity of 14.4m/s.Due to the decrease in the cross-sectional area of the bionicjoint, the flow rate of the drilling fluid increases. As the dril-ling fluid passes through the bionic joint area, the flow areaincreases gradually, and the flow rate of the drilling fluiddecreases. From the velocity vector diagram, it can be seenthat, during the process that the drilling fluid enters fromthe inlet and flows through the bionic joint, the drilling fluidflows steadily along the positive direction of Y , while whenthe drilling fluid passes through the bionic joint and entersinto a larger cross section, the swirling vortex phenomenonoccurs near the wall surface of the pipe, and the flow field isdisturbed.

Figure 12 shows the curve of drilling fluid velocity varia-tion on the central axis of the bionic drill pipe couplingmodel. The flow velocity of the drilling fluid in the flow chan-nel corresponding to the bionic joint is relatively large, andthe flow rate is gradually increasing. As the drilling fluid

3

12

1.06e+06

Contour-1Static pressure

1.01e+069.57e+059.05e+058.52e+057.99e+057.47e+056.94e+056.41e+055.88e+055.36e+05

(Pascal)

1.07e+06

Vector-2Static pressure

1.01e+069.48e+058.88e+058.29e+057.69e+057.10e+056.50e+055.90e+055.31e+054.71e+05

(Pascal)

Figure 13: Drilling fluid pressure cloud diagram and vector diagram on the axis cross section of the coupled model.

7Geofluids

flows out of the bionic joint, the flow channel area increasesand the flow rate of the drilling fluid decreases sharply. Atthe position where the drilling fluid whirlpool occurs (posi-tion no. 3 in Figure 11), the amplitude of the decrease ofthe drilling fluid flow rate slows down.

Figure 13 shows the drilling fluid pressure cloud diagramand vector diagram along the Y-axis center cross section ofthe bionic drill pipe coupling model. As the drilling fluidenters into the flow channel of the bionic joint from the inlet,the flow channel area decreases and the flow field pressuredecreases. After the drilling fluid flows through the cross sec-tion of the bionic joint, the pressure of the flow field increasesgradually until reaching the set outlet pressure value. As canbe seen from the pressure vector diagram, in the process ofdrilling fluid flowing from the inlet into the bionic joint sec-tion, the pressure of the flow field is stable, while when thedrilling fluid passes through the bionic joint and enters alarger flow cross section, pressure disturbance occurs on thewall of the pipe.

Through the simulation analysis, it can be seen that thedrilling fluid flow field is stable at the bionic drill pipe joint,and there is no flow directed to the wall of the joint, whichcan effectively avoid the risk of leakage when the drilling fluidflows through the bionic drill pipe joints. Therefore, the flowperformance of the designed bionic joint is reliable; the dril-ling fluid velocity field and pressure field are stable, whichcan meet the application requirements.

5. Conclusions

(1) Based on the principle of engineering bionics, theresearch on the bionic structure of drill pipe jointsis carried out. For the bionic drill pipe joint, the frag-ile part of the root of the external thread is presplitinto two parts: the torsion shoulder part and the frontthread part, and then, the cushion layer is injectedinto the cutting seam between these two parts. There-fore, it is feasible to separate the torsion transmissionfunction of the drill pipe joint from the connectionfunction and realize the bionic effect of “breakingthe bones and connecting the ribs”

(2) The processing and production of the bionic jointhave been studied, and the test results of the bionicdrill pipe joint show that the tensile performancecan reach 555.48 kN, and the torsion resistance canreach 8914.13N·m, which can meet the performancerequirements

(3) Flow field characteristics of the bionic drill pipe areanalyzed through numerical simulation, simulationresults show that the velocity field and pressure fieldof the fluid are stable, and there is no leakage phe-nomenon at the connecting components of the bionicjoint

Data Availability

The data used to support the findings of this study are avail-able from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (42072345) and the Natural ScienceFoundation of Shaanxi Province (2021 JQ-952).

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